1. Field of the Invention
This application relates to the field of detection of chemical substances and, more particularly, to improving the transport of vapors of chemical substances through tubing.
2. Description of Related Art
Chemical detection may be performed by a variety of detection instruments, such as a gas chromatograph, an ion mobility spectrometer, a mass spectrometer, and/or a differential mobility spectrometer. Many of these chemical detectors require that a chemical sample be vaporized and transported through a length of tubing (the “sample tube”) to reach the detection component. In order to avoid condensation of chemical species contained within the vapor sample (the “sample gas”), the walls of the sample tube may be heated.
In some cases, the process of taking a sample begins with an operator rubbing an absorbent substance (the “sample trap”), such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on the absorber. This sample trap is then brought to the vicinity of the sampling orifice of the chemical detector. The quantity of particles of the target substance on the target surface is usually very small, often corresponding to only nanograms or even picograms of particles per square centimeter. The chemical detector must be very sensitive to identify a valid signal from evaporated target molecules when the initial concentration and surface area of target particles is so small compared to possible background contamination.
A known technique for vaporizing the sample that is commonly employed is to provide a heated zone (the “desorber”) that resembles an oven. The desorber may completely enclose the sample trap when operating or may have an open entrance to facilitate the insertion of a sample trap. A gas pump draws the sample gas from the desorber into the sample entrance orifice of the chemical detector through a sample tube. A second embodiment for chemical species that have a sufficiently high vapor pressure at room temperature is to only provide a sample tube in pneumatic communication with the external environment and the sample entrance orifice of the chemical detector together with a gas pump to draw sample gas into the chemical detector.
The chemicals that may be drawn through the sampling tube may have a wide variety of physical properties. In particular, the most significant properties for this discussion are the vapor pressure as a function of temperature, the rate of adsorption on the inside surface of the sampling tube as a function of temperature, and the decomposition rate at the temperature of the sampling tube.
The vapor pressure is important, because a low vapor pressure substance is more likely to partially condense on the inside surface of the sampling tube when a concentrated pulse of sample gas passes through the sampling tube. Subsequent slow re-evaporation of the temporarily condensed sample gas leads to equipment downtime until all previous chemical substance has been purged from the chemical detector system.
The substance that comprises the inside surface of the sampling tube may have greater or less affinity for adhesion (“adsorption”) of a specific chemical in the sample gas. Adsorption is a function of the temperature as well as the substance of the inside surface of the sampling tube. In general, higher temperature leads to less adsorption.
Decomposition of the chemical can potentially create new chemicals and deposit low vapor pressure decomposition products on the inside surface of the sampling tube. Sufficient temperature can fully decompose most organic materials to substances with virtually no vapor pressure. Decomposition may also undesirably selectively remove a target gas species from the sample gas, and this limits the maximum temperature that is acceptable for the sample tube when sample gas is flowing through it.
Rapid purging of unwanted sample gas chemicals from the inside of the sampling tube is significant for minimizing the time required to return a chemical detector to its “ready” condition. If too hot a sampling tube is employed, decomposition into unwanted substances may occur and even decompose chemical species of interest. If too cold a sampling tube is employed, excessive condensation and lengthy purging time will be required. Normally, the temperature of the sampling tube may be a compromise between these two extremes, and ambient chemical species will eventually be encountered that lead to an undesirably long purging time and consequently a lengthy “down time” of the chemical detection instrument.
Accordingly, it would be desirable to provide a system that addresses the above-noted issues and improves the processes related to transporting vapors of chemical substances through tubing.